Sub-1 nm: A Critical Feature Size in Materials Science

ConspectusSize is one of the most important issues in materials science. Sizes of conventional nanomaterials are usually one to one hundred nanometers and, according to size effect, nanomaterials usually exhibit different optical, electrical, magnetic, thermal, and catalytic properties compared with bulk materials. As the important transition from atomic scale to nanoscale, subnanometer scale is a critical feature size in materials science. With the size of nanomaterials decreasing to subnanometer scale, they further show some distinctive properties. First, subnanometer materials (SNMs) show ultrahigh specific surface area and nearly 100% surface atomic ratio, greatly enhancing their interaction with external fields. Second, the intermolecular interaction between SNMs can dominate their self-assembly process, similar to the self-assembly process of some biomacromolecules. Third, due to the dimensional similarity, one-dimensional (1D) SNMs exhibit polymer-analogue properties, endowing them easy processability. And thus, SNMs not only show excellent optical, catalytic, adhesive properties, etc. but also can be processed into functional fabrics, films, gels, and so on. In order to construct functional SNMs, a good/poor solvent system and cluster–nuclei coassembly strategy are developed, which are facile and widely applicable. Until now, various SNMs with different compositions, morphologies, and structures have been prepared successfully through these two methods, and temperature-scale and large-scale preparation are realized for some SNMs, laying a solid foundation for practical applications of SNMs. Based on the cluster–nuclei strategy, multiple effective components can be combined into the block copolymer-analogue structures, and the performance can be improved due to the synergistic effect. And through assembling polyoxometalate (POM) clusters into clusterphene structures, electron delocalization may be realized to greatly enhance the catalytic performance. This Account summarizes some work of our group on SNMs, including structures, polymer-analogue properties, assembly, processing, and catalytic performance. In the structure part, the intrinsic chirality, block copolymer-analogue structure, and metal cation-bridging-POM anion structure are mainly introduced. Then the polymer-analogue properties of 1D SNMs are briefly discussed, including the dispersion, gelation, and adhesion properties. Subsequently, the self-assembly, electrospinning, and wet-spinning of SNMs and surfactant-assisted assembly of POM clusters etc. are introduced. Finally, tailoring catalytic properties in subnanometer scale through synergetic effects and possible electronic delocalization is discussed, which is based on the cluster–nuclei coassembly strategy and surfactant-assisted assembly of POM clusters. The SNMs show great potential in catalysis, optics, adhesion, gelation, and so on, paving a new path for the applications of nanomaterials. This Account briefly introduces the current progress and future challenges in the field of SNMs and we hope this Account will be informative for researchers interested in SNMs and give new inspiration to them.